Editing Rocket engine (section)

===Propellant efficiency===
{{See also|Specific impulse}}
[[Image:Nozzle de Laval diagram.svg|thumb|right|upright|Typical temperature (T), pressure (p), and velocity (v) profiles in a de Laval Nozzle]]
For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created on the walls of the chamber and nozzle by a specific amount of propellant; as this is the source of the thrust. This can be achieved by all of:

* heating the propellant to as high a temperature as possible (using a high energy fuel, containing hydrogen and carbon and sometimes metals such as [[aluminium]], or even using nuclear energy)
* using a low specific density gas (as hydrogen rich as possible)
* using propellants which are, or decompose to, simple molecules with few degrees of freedom to maximise translational velocity

Since all of these things minimise the mass of the propellant used, and since pressure is proportional to the mass of propellant present to be accelerated as it pushes on the engine, and since from [[Newton's third law]] the pressure that acts on the engine also reciprocally acts on the propellant, it turns out that for any given engine, the speed that the propellant leaves the chamber is unaffected by the chamber pressure (although the thrust is proportional). However, speed is significantly affected by all three of the above factors and the exhaust speed is an excellent measure of the engine propellant efficiency. This is termed ''exhaust velocity'', and after allowance is made for factors that can reduce it, the '''[[effective exhaust velocity]]''' is one of the most important parameters of a rocket engine (although weight, cost, ease of manufacture etc. are usually also very important).

For aerodynamic reasons the flow goes sonic ("[[Choked flow|chokes]]") at the narrowest part of the nozzle, the 'throat'. Since the [[speed of sound]] in gases increases with the square root of temperature, the use of hot exhaust gas greatly improves performance. By comparison, at room temperature the speed of sound in air is about 340&nbsp;m/s while the speed of sound in the hot gas of a rocket engine can be over 1700&nbsp;m/s; much of this performance is due to the higher temperature, but additionally rocket propellants are chosen to be of low molecular mass, and this also gives a higher velocity compared to air.

Expansion in the rocket nozzle then further multiplies the speed, typically between 1.5 and 2 times, giving a highly [[collimated]] hypersonic exhaust jet. The speed increase of a rocket nozzle is mostly determined by its area expansion ratio—the ratio of the area of the exit to the area of the throat, but detailed properties of the gas are also important. Larger ratio nozzles are more massive but are able to extract more heat from the combustion gases, increasing the exhaust velocity.